Type I interferons (IFNs) are pleiotropic cytokines with important antineoplastic and antiviral activities and constitute key elements in the immune surveillance against cancer and viral infections. Although several interferon stimulated genes (ISGs) that mediate the antiviral properties of IFNs have been identified, very little is known on the genes and protein products that mediate generation of the antiproliferative and antineoplastic effects of IFNs. We have identified a novel family of ISGs, the family of SLFN genes. Our data have provided the first evidence that engagement of the Type I IFN receptor (IFNR) results in expression of mouse and human SLFN genes, and have demonstrated that members of this family are involved in the suppression of anchorage-independent malignant cell growth and inhibition of collagen invasion by melanoma cells. The current proposal is a systematic approach to define the roles of distinct SLFN genes in suppression of oncogenesis and in the generation of the antineoplastic effects of IFNs; and to examine whether defects in the expression of specific SLFNs correlates with IFN-resistance in vitro and in vivo. Specific aim 1 will define the mechanisms of regulation of expression of SLFNs and will dissect the functional roles of distinct SLFNs in IFN-induced growth inhibitory effects. Studies to define effectors and mediators of responses downstream of SLFNs will be also performed. Specific aim 2 will determine whether induction of expression of SLFN proteins is essential for generation of the antineoplastic effects of IFN? in solid tumor mouse models in vivo. For that purpose malignant melanoma or renal cell carcinoma (RCC) mouse models will be established using mice with the elektra phenotype (Slfn2eka/eka) or Slfn1 or Slfn3 knockout mice. Finally, specific aim 3 will examine whether defective expression of human SLFNs correlates with resistance to the antineoplastic properties of IFN1 on primary malignant cells from patients with malignant melanoma or renal cell carcinoma (RCC), malignancies known to be responsive to immune modulation and IFN-treatment. Altogether, the proposed studies will address important issues on the role of SLFNs in the generation of antitumor responses and the means by which malignant cells develop resistance to IFNs.